Engines can be coupled to emission control devices, such as catalytic converters, to reduce exhaust emissions. However, these devices can become contaminated with sulfates, for example. In order to remove these contaminates, the temperature of the emission control device is raised significantly and a near stoichiometric air-fuel ratio is provided that alternates, or oscillates, around stoichiometry (between lean and rich).
One type of engine exhaust system routes all of the engine cylinders into a single exhaust path. One approach for raising temperature of such a single exhaust path sequentially operates some cylinders lean, and then some rich to create heat. Such an approach is described in U.S. Pat. No. 5,974,788, for example.
The inventors herein have recognized numerous disadvantages with such an approach In particular, even with ignition timing adjustments, it is difficult to reduce torque disturbances due to the air-fuel ratio transitions across stoichiometry from significantly lean values to significantly rich values. Furthermore, heat generated in the emission control device is a function not only of oxidant storage, but also of the modulation frequency and heat carried from the exhaust gasses to the emission control device.